> In the same way as no-one should ever try and create Unity from scratch
I don't think anyone is really trying to create Unity from scratch (unless they're trying to compete with Unity). But that doesn't necessarily mean you should just use Unity. Unity is complicated, and designed to be a general purpose game engine. Which means for any given game, it may or may not be a good fit. A lot of times you can get up and running faster in Unity, but soon start to hit walls as you try to do something more advanced. If you wrote your own engine, you would just modify it to meet your needs, but with Unity you end up trying to find workarounds for its shortcomings.
I also think your example with React is flawed. There's a big difference between left-pad and React, and I think the most interesting packages to discuss is the range of things between those two.
I don't think anyone is really trying to create Unity from scratch (unless they're trying to compete with Unity). But that doesn't necessarily mean you should just use Unity. Unity is complicated, and designed to be a general purpose game engine. Which means for any given game, it may or may not be a good fit. A lot of times you can get up and running faster in Unity, but soon start to hit walls as you try to do something more advanced. If you wrote your own engine, you would just modify it to meet your needs, but with Unity you end up trying to find workarounds for its shortcomings.
I also think your example with React is flawed. There's a big difference between left-pad and React, and I think the most interesting packages to discuss is the range of things between those two.